1. Field of the Invention
The present invention relates to wireless communication, and more particularly, to a method for determining a packet data rate in a wireless communication according to a channel status and a data packet reception status.
2. Discussion of the Related Art
In the transmission of packet data in a mobile communication system, the data rate is determined through a calculation based on dynamic channel conditions, which may be expressed in terms of a packet error rate resulting from a recent instance of transmission. In doing so, channel conditions are repeatedly monitored and the packet data rate is continuously updated, to maintain an acceptable packet error rate for the fastest data transfer permissible. In other words, favorable channel conditions permit higher data rates while poor channel conditions necessitate lower data rates.
The packet data rate is determined based on channel status information, which corresponds to a calculated maximum rate permissible for subsequent data to be transmitted, and the packet data rate is set by a data scheduler accordingly. The packet data rate may be determined at a base station, whereby the channel status information is transmitted to the base station, which performs a calculation determining the maximum rate. The packet data rate may also be determined at a mobile terminal, whereby the calculation of the maximum rate is performed by the mobile station and a calculated maximum value is transmitted to the base station. In either case, the packet data rate is calculated based on received information and is relevant to conditions felt at the receiving side.
The optimal packet data rate is ultimately determined by referencing a set of packet data rates stored in a lookup table, where each data rate corresponds to a range of values of the channel status information, each of which is separated by a boundary value. Accordingly, once the channel status information is obtained—typically, as a signal-to-interference-plus-noise ratio—the optimal packet data rate can be determined. The selected rate, however, represents a conservative assessment. That is, the selected packet data rate corresponds to the range immediately below that corresponding to the obtained channel status information.
FIG. 1 illustrates a channel environment existing at the time of data transmission between a mobile terminal and base station. Here, the mobile terminal first transmits channel information or a requested packet data rate to the base station at a time A, and the base station thereafter transmits the packet data to the mobile station at a time B.
Meanwhile, the data scheduler requires a delay time ts for preparation of the data packet. Thus, in addition to varying propagation delays, there is a marked differential in the time (A) of sensing the channel status at the mobile and the time (B) of transmitting the data from the base terminal. Since the channel conditions are in a continual state of flux and since the mobile terminal is likely to be in a state of motion, there is a difference between the channel status at time B and the channel status at time A, and as indicated by the example plot of the channel information of FIG. 1, this difference may be great. Moreover, in the case of high-speed motion by a mobile terminal user, say, by vehicular means, an accurate prediction of channel status becomes more critical. Therefore, when determining a packet data rate based on quality of service, there is an increased likelihood of transmission failure.
If, based on inaccurate assumptions of the channel conditions, the selected packet data rate is too high, the packet error rate will exceed acceptable limits such that the quality of service suffers. On the other hand, selection of a packet data rate that is too low results in an inefficient uplink or downlink, and the quality of service still suffers. Therefore, a method of determining an optimal packet data rate for selection is needed to accurately reflect changes in the channel environment.